1. Field of the Invention
The present invention is directed to a medical device, and a method for operating the device, implantable in the body of a patient for stimulating tissue contractions, such as a cardiac pacemaker.
2. Description of the Prior Art
A pacemaker is known, and is described in the publication "DIALOG-Schrittmacher 728-Gebrauchsanweisung," October, 1986, Siemens-Elema AB, Solna, Sweden, which includes means for detecting spontaneous tissue contractions, means for stimulating tissue contractions and control means. The control means control the generation of stimulation pulses by the means for stimulating tissue contractions so that after the expiration of a first time interval, having an adjustable duration initiated by the detection or the stimulation of a tissue contraction, the means for stimulating is activated as long as a spontaneous tissue contraction, which initiates a new first time interval, has not been detected before the expiration of the first time interval. The detection of a spontaneous tissue contraction is prevented from starting a new first time interval during a second time interval which is initiated by the detection or stimulation of a tissue contraction, and which has a duration shorter than the duration of the first time interval. A new second time interval is initated upon the detection of a spontaneous tissue contraction during the second time interval.
In this known device, the duration of the first time interval, referred to as the base interval, corresponds to that stimulation frequency with which the device stimulates the heart in the absence of spontaneous heartbeats. In this known heart pacemaker, the duration of the first time interval as well as that of the second time interval are adjustable by a programming event independently of one another. The second time interval, referred to as the refractory time, is composed of two time segments. During the first segment of the second time interval, referred to as the absolute refractory time and whose duration is likewise adjustable on the basis of a programming event, the means for detecting are completely insensitive to any and all cardiac signals. This prevents tissue depolarizations occurring after the heart is charged with a stimulation pulse from causing mis-detections, i.e. from being incorrectly detected as a spontaneous heartbeat by the means for detecting. In the second, following segment of the second interval, referred to as the relative refractory time, spontaneous heartbeats that occur are in fact detected but a new base interval does not begin to run. Instead, the refractory time is again started over its full length. When, thus, the heart spontaneously beats so fast that the spontaneous heartbeats respectively appear within the relative refractory time, i.e., during the second segment of the second time interval, the known heart pacemaker switches from the VVI mode in which it normally operates into the VOO mode that corresponds to the asynchronous operating mode. The known heart pacemaker then stimulates the heart independently of the spontaneously occurring heartbeats with a stimulation frequency that corresponds to the duration of the base interval. This is advantageous since certain reentry tachycardia can thereby terminated. Mis-detections occurring during the relative refractory time as occur, for example, given a sensitivity of the means for detection that is set too high, moreover, can not start a new base interval. The appearance of misdetections can thus not prevent a stimulation of the heart that is required.
Based on the above, it is preferable to set a comparatively long refractory time in this known heart pacemaker. This has the consequence in practice, however, that the spontaneous heartbeat rate can rise to such an extent under a physical stress that the known heart pacemaker switches into its asynchronous operating mode, i.e. into the VOO mode, without a physiological reason for this switch being present. The heart of the patient is thus stimulated without this being actually required. This can lead to disturbances in heart rhythm. At any rate, the patient in whom the heart pacemaker is implanted is physically uncomfortable. Only an extremely experienced physician is in the position-taking the individual requirements and peculiarities of the respective patient into consideration-of programming the duration of the refractory time for a defined base interval such that, first, injurious consequences of mis-detections and external disturbances are avoided and, second, a physiologically unfounded transition of the heart pacemaker into its asynchronous operating mode is impossible. The above considerations also analogously apply to heart pacemakers working the SSI or DDD mode, whereby the risk of the physiologically unfounded transition respectively into the SOO or AOO mode is present.